On-vehicle night vision devices, security systems, and the like include infrared sensors for use to detect living bodies at night. To sense infrared rays with wavelengths of about 8 to 12 μm emitted from living bodies, such an infrared sensor is provided, in front of the sensor section, with an optical element, such as a filter or a lens, capable of transmitting infrared rays in the above wavelength range.
Examples that can be cited as a material for the optical element as described above include Ge and ZnSe. These materials are crystalline and therefore poor in processability, so that they are difficult to process into complicated shapes, such as an aspheric lens. This makes mass production difficult and also presents a problem of difficulty in size reduction of the infrared sensor.
To cope with the above, chalcogenide glasses are proposed as vitreous materials that can transmit infrared rays with wavelengths of about 8 to 12 μm and are relatively easy to process (see, for example, Patent Literature 1 or 2).